1. Field of the Invention
The present invention relates to coated optical fibers whose radiation-curable primary coatings are easy to cleanly strip from the optical fiber.
2. Background Discussion
Optical fibers for light transmission as drawn are exceptionally strong and have very few intrinsic defects. However, even a small surface flaw can render such a fiber brittle and easily broken. Thus, such fibers are generally coated by a primary and optionally a secondary coating for protection as disclosed by U.S. Pat. Nos. 5,352,712, 5,527,835 and 5,538,791 to Shustack, these patents being incorporated herein by reference in their entirety. However, when it is desired to splice optical fibers, these coatings are stripped away to expose the optical fibers so they may be spliced. The removal of these coats, as a preparatory step prior to splicing, entails the risk of scratching or otherwise damaging the optical fiber.
A. Optical Fiber Structure
The primary coating is applied directly to the glass fiber and, when cured, forms a soft, rubbery, compliant material which serves as a buffer to cushion and protect the fiber by relieving the stresses created when the fiber is bent, cabled or spooled. Such stress might otherwise induce microbending of the fibers and cause attenuation of the light traveling through them, resulting in inefficient signal transmission. The secondary coating is applied over the primary coating, and functions as a hard, tough protective outer layer, preventing damage to the glass fiber during processing and use. More than one optical fiber can be combined and embedded in a matrix.
Certain characteristics are desirable for the primary coating layer. For example, it must maintain adequate adhesion to the glass fiber during thermal and hydrolytic aging, yet be strippable therefrom for splicing purposes. The tensile modulus of the primary coating must be low to cushion and protect the fiber by readily relieving the stresses on the fiber which can induce microbending and consequent inefficient signal transmission. This cushioning effect must be maintained through the temperature range to which the fiber may be exposed throughout its lifetime. Thus, it is necessary for the primary coating to have a low glass transition temperature (Tg). This low glass transition temperature will ensure that the coating remains in its rubbery state throughout the possible use temperature range.
The primary coating should also have a relatively high refractive index, i.e., greater than that of the cladding material of the fiber to be coated. This high refractive index allows for a refractive index differential between the glass cladding and the primary coating. This differential allows errant light signals to be refracted away from the glass core.
Another requisite quality of the primary (buffer) coating is resistance to moisture or other solvents, e.g., hydrocarbons. Moisture will rapidly degrade the strength of the coating itself as well as the underlying glass fiber under stress.
Another important property of optical fiber coatings is that, when cured, they have a low content of unbound material. While ultraviolet curable materials are often referred to as 100% solids, they may still contain a significant amount of chemically unbound material after the ultraviolet cure. This unbound material can be extractable with solvent or water, or it can be volatile under certain conditions. The presence of an extractable or volatile component in optical fiber products can cause problems which may be detrimental to the fibers. Such potential problems may manifest themselves throughout the lifetime of the optical fiber.
B. Optical Fiber Stripping Can Cause Problems
Although coated optic fibers are very well made, there is often a need to splice together such fibers. This requires stripping the coatings from the fibers and thus exposes the fibers to risk as discussed above. Residue left of the fibers after stripping contributes to the risk. Thus, in the art of optic fibers, there is a significant need to facilitate splicing of optical fibers, especially ribbons of parallel optic fibers, by providing fiber optic coatings which are cleanly strippable to leave little or no residue.
In ribbon stripping, blades of a heated tool cut through the matrix, ink layer, secondary coating and primary coating of the fiber almost to the glass fiber. It is noted that ink is optional or the ink can be avoided by employing a pigmented secondary coating. Then the blades are pulled longitudinally away from the fiber optic ribbon to remove the matrix, secondary coating and primary coating surrounding a portion of the glass of the ribbon. However, it is difficult to remove the matrix and coatings without leaving some residue of the primary coating on the glass. Sometimes the residue can be removed by wiping the glass with an alcohol-laden piece of cloth or paper, e.g., CHEMWIPE towelettes. However, the residue must be removed because the applied splicing coating must adhere directly to the optic fiber. Mechanical wiping of the residue from the glass runs the risk of scratching, and thus weakening, the glass.
This residue is especially significant for fiber optic ribbons provided with full peel matrices. Fiber optic ribbons comprise a plurality of parallel optic fibers held together by a matrix. The full peel matrix is easily removable because it easily peels away from the optic fibers. However, it is generally more difficult to cleanly strip optic fiber ribbons employing a full peel matrix.
The full peel matrix peels away easily because it does not strongly adhere to the (i) secondary coating or (ii) ink which in turn adheres to the secondary coating which in turn adheres to the primary coating. In embodiments employing non-full peel matrices, the coatings and matrices are separated after applying a solvent to the matrix. This residue problem also is present for optical ribbons provided with the non-full peel matrices.
It would be desirable to provide a primary coating which facilitates simultaneous stripping of the matrix, secondary and primary coating while leaving little or no residue on the stripped glass core.